I had a water stain appear in my garage right near where the main AC trunk comes up the center of the house. Of course at the time I did not know this I thought i had a leaking pipe. After I opened up the ceiling I found that it was a duct that was leaking. The insulation was completely soaked. I took off the insulation and the duct was rusting away. I guess this has been a problem since I bough the house. The builder said its because they did not spray foam around the joints where the duct cuts through the joints. I took down the dry wall to inspect the rest of the ducts and looks like there are several places where they did not tape it well and there are small gaps where hot air can get to the duct. What should I do to fix this? The insulation is R8.3 fiberglass. Should I take all that out and wrap the duct in a better type of insulation? The rust did not eat through the duct so I hope I don't have to replace any of it. Any guidance you could provide on the best course of action would be greatly appreciated. The house is out of warranty and my insurance considers this a maintenance issue so I am on my own to pay for this fix.enter image description here
Air seal with air duct sealant (a gray goo that hardens after it's brushed on or applied with a putty knife), not duct tape, and insulate, preferably a LOT better than R8 (your heating/cooling cost is hugely affected by poorly insulated & leaky ducts in unconditioned space.)
You can reinforce the duct sealant by embedding fiberglass mesh drywall tape in it. I don't suggest using duct tape because it has a habit of falling off after a few years.